My unprofessional opinion is that it is time to check your friend list and who has been inside your garage. Less people the better. Also hate to say it but trust NOBODY! Mind you i also don't want to see you becoming that ole coot that bitches about everyone. Invest in a better lockup strategy and get some motion sensor lights with super high wattage bulbs. Also not sure on your area at all but a fence around your yard? (natural deterrent to potential theives) Also lets you know that they purposely crossed into your yard if found on wrong side of fence. The problem with putting up a sign warning about a dog or anything if something were to happen the lawyers will jump all over that sign saying that "you knew your dog was aggressive because of the sign" Ever find someone on your yard, have them charged with trespassing and the word will get around.